Fuel cell system

ABSTRACT

A fuel cell system includes a fuel cell stack and a control device that controls operation of the fuel cell system based on a measured voltage value measured by a voltage sensor. When the fuel cell system is started and a value measured by a temperature sensor is equal to or less than a temperature determined in advance, the control device raises the voltage of the fuel cell stack until a voltage condition determined in advance is met, by supplying a cathode with an oxidant gas before current sweep is started. The control device sets a voltage command value and a current command value such that an operation point of the fuel cell stack is on an equal power line of the fuel cell stack when the operation point is caused to transition in at least a part of a transition period.

INCORPORATION BY REFERENCE

The disclosure of Japanese Patent Application No. 2020-074653 filed onApr. 20, 2020 including the specification, drawings and abstract isincorporated herein by reference in its entirety.

BACKGROUND 1. Technical Field

The present disclosure relates to the technology of a fuel cell system.

2. Description of Related Art

There has hitherto been known a fuel cell system in which warm-upoperation is performed with the amount of an oxidant gas supplied to acathode reduced compared to that during normal power generation (e.g.Japanese Patent No. 6187774).

SUMMARY

In the related art, when the fuel cell system performs the warm-upoperation, an upper limit current and a lower limit current are set, anda current target value is set as a current command value such that thecurrent target value falls within the range between the upper limitcurrent and the lower limit current when the current target value is outof the range between the upper limit current and the lower limitcurrent. In the related art, in addition, the upper limit current is setusing, as an index, a current value which is limited in order to limit arise in the concentration of hydrogen to be discharged from a fuel cellstack due to pumping hydrogen. In a transition period until a transitionis made to a target operation point determined by the target currentvalue and a target voltage value for the fuel cell stack, however, anoxidant gas may be short and pumping hydrogen may be generated at acathode of fuel cells. When pumping hydrogen is generated in the fuelcells, a sufficient oxidant gas is not supplied to the surface of acatalyst of the cathode because of the pumping hydrogen, and thereforeit is highly likely that pumping hydrogen is also generated thereafter.When pumping hydrogen is generated at the cathode, the concentration ofhydrogen in a gas discharged from the cathode may be high. In addition,the fuel cell system includes a secondary battery to be charged with anddischarge power generated by the fuel cell stack. The warm-up operationis executed when the outside air temperature is below the freezingpoint, for example. The allowable range of the charge/discharge amountmay be limited at a temperature below the freezing point, depending onthe type of the secondary battery. Hence, when the warm-up operation isexecuted at a temperature below the freezing point, it is occasionallydifficult to control the charge/discharge amount of the secondarybattery within the allowable range, that is, within a certain range. Theterm “pumping hydrogen” refers to hydrogen generated at the cathode byrecombination of hydrogen ions and electrons conducted from an anodebecause of the shortage of oxygen at the cathode during warm-upoperation.

The present disclosure can be implemented in the following aspect.

An aspect of the present disclosure provides a fuel cell system. Thefuel cell system includes: a fuel cell stack that has a plurality ofstacked fuel cells each having an anode and a cathode; a voltage sensorconfigured to measure a voltage of the fuel cell stack; an oxidant gassupply system configured to supply the cathode with an oxidant gascontaining oxygen; a fuel gas supply system configured to supply theanode with a fuel gas; a temperature sensor configured to measure atemperature related to the fuel cell system; a secondary batteryconfigured to be charged with power generated by the fuel cell stack anddischarge the power; and a control device configured to controloperation of the fuel cell system based on a measured voltage valuemeasured by the voltage sensor, in which: the control device isconfigured to, when the fuel cell system is started and a value measuredby the temperature sensor is equal to or less than a temperaturedetermined in advance, raise the voltage of the fuel cell stack until avoltage condition determined in advance is met, by causing the oxidantgas supply system to operate and supplying the cathode with the oxidantgas before current sweep from the fuel cell stack is started, andexecute warm-up operation in which a temperature of the fuel cell stackis raised, by starting the current sweep when the measured voltage valuemeets the voltage condition; and the control device is configured to,when executing the warm-up operation, set a voltage command value and acurrent command value such that an operation point determined by avoltage value and a current value of the fuel cell stack is on an equalpower line of the fuel cell stack, which indicates the same generatedpower as required generated power of the fuel cell stack, when theoperation point is caused to transition in at least a part of atransition period from the start of the current sweep until theoperation point reaches a target operation point determined by a targetvoltage value and a target current value during the warm-up operation.With this aspect, warm-up operation can be performed after sufficientoxygen is provided to the cathode of the fuel cell stack, by startingcurrent sweep after the measured voltage value meets a voltage conditiondetermined in advance. Consequently, it is possible to reduce thepossibility that pumping hydrogen is generated because of the lack ofoxygen at the cathode during warm-up operation. Moreover, the controldevice sets the voltage command value and the current command value suchthat the operation point is on the equal power line when the operationpoint is caused to transition in the transition period, and thusdeviation of the actual power generated by the fuel cell stack from therequired generated power can be suppressed. Consequently, thecharge/discharge amount of the secondary battery can be controlledwithin a certain range. With the above aspect, as described above, thecharge/discharge amount of the secondary battery can be controlledwithin a certain range while reducing the possibility that pumpinghydrogen is generated.

In the aspect described above, the control device may be configured to:execute normal current control in which the current command value israised to the target current value at a proportion determined in advancein a post-switch period, of the transition period, from a time when themeasured voltage value has become equal to or less than a switchingvoltage value determined in advance until a time when the measuredvoltage value reaches the target voltage value of the target operationpoint; suspend the normal current control and execute stand-by controlin which the current command value is kept constant when the normalcurrent control is executed and the measured voltage value reaches acontrol start voltage value which is less than the voltage commandvalue; and end the stand-by control and resume the normal currentcontrol by permitting a change in the current command value when themeasured voltage value reaches a permission voltage value which is equalto or more than the voltage command value during execution of thestand-by control. With this aspect, in the post-switch period, thenormal current control is executed, and thus execution of excessivecurrent sweep can be suppressed. With this aspect, in addition, theshortage of oxygen at the cathode can be suppressed by executing thestand-by control when the normal current control is executed and themeasured voltage value reaches the control start voltage value. Thus,generation of pumping hydrogen can be suppressed.

In the aspect described above, the voltage sensor may be configured tomeasure a total voltage of the fuel cell stack; and the voltagecondition may be a condition that a value of the total voltage as themeasured voltage value measured by the voltage sensor has become morethan a reference voltage value determined in advance. With this aspect,it is possible to determine, based on the value of the total voltage ofthe fuel cell stack, whether the voltage condition determined in advanceis met.

In the aspect described above, supply of the oxidant gas to the fuelcell stack may be performed on a side of a first end portion of the fuelcell stack in a stacking direction in which the fuel cells are stacked;the voltage sensor may be configured to measure a voltage of an endportion-side fuel cell which is a fuel cell among the fuel cells, theend portion-side fuel cell being positioned on a side of a second endportion which is opposite from the side of the first end portion; andthe voltage condition may be a condition that a voltage value of the endportion-side fuel cell as the measured voltage value measured by thevoltage sensor has become more than an end portion-side referencevoltage value determined in advance. With this aspect, generation ofpumping hydrogen can be further suppressed by determining whether thevoltage value on the side of the second end portion meets a voltagecondition determined in advance, even if the fuel cell stack is so longin the stacking direction that it takes a considerable time for theoxidant gas to reach the side of the second end portion. Supply of theoxidant gas to the side of the second end portion of the fuel cell stackis delayed compared to the side of the first end portion of the fuelcell stack, and therefore the voltage rise on the side of the second endportion due to the supply of the oxidant gas tends to be delayedcompared to the side of the first end portion. Hence, generation ofpumping hydrogen can be further suppressed by determining whether thevoltage value on the side of the second end portion, the voltage rise onwhich tends to be delayed, meets the voltage condition determined inadvance.

The present disclosure can be implemented in a variety of forms. Besidesthe fuel cell system described above, the present disclosure can beimplemented in the form of a control method for the fuel cell system, acomputer program that causes a computer to execute the control method, anon-transitory storage medium that stores the computer program, etc.

BRIEF DESCRIPTION OF THE DRAWINGS

Features, advantages, and technical and industrial significance ofexemplary embodiments of the disclosure will be described below withreference to the accompanying drawings, in which like signs denote likeelements, and wherein:

FIG. 1 illustrates a schematic configuration of a fuel cell system;

FIG. 2 illustrates a detailed configuration of the fuel cell system;

FIG. 3 is a conceptual diagram illustrating the electrical configurationof the fuel cell system;

FIG. 4 is an internal block diagram of a control device;

FIG. 5 indicates the temperature properties of a secondary battery;

FIG. 6 is a flowchart illustrating a start process for the fuel cellsystem;

FIG. 7 is a flowchart illustrating an operation point transitionprocess;

FIG. 8 is a first chart illustrating the relationship between thevoltage and the current of the fuel cell stack;

FIG. 9 is a second chart illustrating the relationship between thevoltage and the current of the fuel cell stack; and

FIG. 10 is a flowchart illustrating a start process for a fuel cellsystem 10 according to a second embodiment.

DETAILED DESCRIPTION OF EMBODIMENTS A. First Embodiment

FIG. 1 illustrates a schematic configuration of a fuel cell system 10.The fuel cell system 10 is mounted on a fuel cell electric vehicle 12,for example, and used as a power generation device that drives a drivemotor of the fuel cell electric vehicle 12. The fuel cell system 10includes a fuel cell stack 116, a fuel gas supply/discharge system 50,an oxidant gas supply/discharge system 30, and a refrigerant circulationsystem 70.

The fuel cell stack 116 includes a plurality of fuel cells 11 and a pairof end terminals 110 and 120. The fuel cells 11 are each in a plateshape, and are stacked in a stacking direction SD corresponding to thethickness direction. The fuel cells 11 are each a solid polymer fuelcell battery supplied with an oxidant gas and a fuel gas as reactiongases to generate power through an electrochemical reaction betweenoxygen and hydrogen. In the present embodiment, the oxidant gas is airwhich contains oxygen, and the fuel gas is hydrogen. The fuel cells 11are each a power generation element that can generate power by itself.The fuel cells 11 each include a membrane electrode assembly and twoseparators that interpose the membrane electrode assembly. The membraneelectrode assembly has an electrolyte membrane, an anode disposed on onesurface of the electrolyte membrane, and a cathode disposed on the othersurface of the electrolyte membrane. An opening portion (notillustrated) is provided at the outer peripheral end portion of each ofthe fuel cells 11 to form a manifold Mfa that allows a flow of thereaction gases and reaction off gases that have passed through powergeneration portions. The manifold Mfa is branched and connected to thepower generation portions of the fuel cells 11. In addition, an openingportion (not illustrated) is provided at the outer peripheral endportion of each of the fuel cells 11 to form a manifold Mfb that allowsa flow of a refrigerant.

The end terminals 110 and 120 are disposed at opposite end portions ofthe fuel cells 11 in the stacking direction SD. Specifically, a firstend terminal 110 is positioned at a first end portion of the fuel cellstack 116, and a second end terminal 120 is positioned at a second endportion, which is opposite from the first end portion, of the fuel cellstack 116. The first end terminal 110 has opening portions 115 formed asthrough holes that form the manifold Mfa and the manifold Mfb. On theother hand, the second end terminal 120 does not have opening portions115 formed as through holes that form the manifold Mfa and the manifoldMfb. That is, the fuel gas, the oxidant gas, and the refrigerant aresupplied to and discharged from only the first side of the fuel cellstack 116 in the stacking direction SD. Fuel cells 11 positioned on theside of the second end portion, among the plurality of fuel cells 11,are also called “end portion-side fuel cells 11 e”. In the presentembodiment, the end portion-side fuel cells 11 e include a fuel cell 11positioned closest to the second end portion.

The fuel gas supply/discharge system 50 has a fuel gas supply function,a fuel gas discharge function, and a fuel gas circulation function. Thefuel gas supply function is a function to supply the fuel gas to theanodes of the fuel cells 11. The fuel gas discharge function is afunction to discharge the fuel gas discharged from the anodes of thefuel cells 11 (also referred to as “fuel off gas”) to the outside. Thefuel gas circulation function is a function to circulate the fuel gaswithin the fuel cell system 10.

The oxidant gas supply/discharge system 30 includes an oxidant gassupply function to supply the oxidant gas to the cathodes of the fuelcells 11, an oxidant gas discharge function to discharge the oxidant gasdischarged from the cathodes of the fuel cells 11 (also referred to as“oxidant off gas”) to the outside, and a bypass function to dischargethe supplied oxidant gas to the outside not via the fuel cells 11.

The refrigerant circulation system 70 circulates the refrigerant throughthe fuel cell stack 116 to adjust the temperature of the fuel cell stack116. Examples of the refrigerant include an antifreezing solution suchas ethylene glycol and a liquid such as water.

FIG. 2 illustrates a detailed configuration of the fuel cell system 10.In FIG. 2, the directions of the fuel gas, the oxidant gas, and therefrigerant which are supplied to the fuel cell stack 116 and dischargedfrom the fuel cell stack 116 are indicated by arrows. The fuel cellsystem 10 has a control device 60, in addition to the fuel cell stack116, the oxidant gas supply/discharge system 30, the fuel gassupply/discharge system 50, and the refrigerant circulation system 70discussed above. The control device 60 controls operation of the fuelcell system 10. The control device 60 will be discussed in detail later.

The oxidant gas supply/discharge system 30 includes an oxidant gassupply system 30A and an oxidant gas discharge system 30B. The oxidantgas supply system 30A supplies the oxidant gas to the cathodes of thefuel cell stack 116. The oxidant gas supply system 30A has an oxidantgas supply path 302, an outside air temperature sensor 38 as atemperature sensor, an air cleaner 31, a compressor 33, a motor 34, anintercooler 35, and a first pressure regulation valve 36.

The oxidant gas supply path 302 is a pipe disposed upstream of the fuelcell stack 116 to communicate between the outside and the cathodes ofthe fuel cell stack 116. The outside air temperature sensor 38 measuresa temperature related to the fuel cell system 10. Specifically, theoutside air temperature sensor 38 measures the temperature of air as theoxidant gas to be taken into the air cleaner 31, that is, the outsideair temperature as the ambient temperature. The result of themeasurement by the outside air temperature sensor 38 is transmitted tothe control device 60. The air cleaner 31 is provided in the oxidant gassupply path 302 upstream of the compressor 33, and removes foreignmatter in the oxidant gas to be supplied to the fuel cell stack 116. Thecompressor 33 is provided in the oxidant gas supply path 302 upstream ofthe fuel cell stack 116, and discharges compressed air toward thecathodes in accordance with an instruction from the control device 60.The compressor 33 is driven by the motor 34 which operates in accordancewith an instruction from the control device 60. The intercooler 35 isprovided in the oxidant gas supply path 302 downstream of the compressor33. The intercooler 35 cools the oxidant gas which has been compressedby the compressor 33 to become hot. The first pressure regulation valve36 is an electromagnetic valve or an electric valve. The first pressureregulation valve 36 regulates the flow rate of the oxidant gas which isdirected from the oxidant gas supply path 302 toward the fuel cell stack116 with the opening degree of the first pressure regulation valve 36regulated by the control device 60.

The oxidant gas discharge system 30B discharges the oxidant gas whichhas flowed through the cathodes to the outside. The oxidant gasdischarge system 30B has an oxidant gas discharge path 308, a bypasspath 306, a second pressure regulation valve 37, and a third pressureregulation valve 39. The oxidant gas discharge path 308 is a pipe thatdischarges, to the outside, the oxidant gas discharged from the cathodesof the fuel cell stack 116 (also referred to as “oxidant off gas”) andthe oxidant gas which has flowed through the bypass path 306. The secondpressure regulation valve 37 is an electromagnetic valve or an electricvalve. The second pressure regulation valve 37 regulates the backpressure in a cathode-side flow path of the fuel cell stack 116 with theopening degree of the second pressure regulation valve 37 regulated bythe control device 60. The second pressure regulation valve 37 isdisposed in the oxidant gas discharge path 308 upstream of a locationwhere the bypass path 306 is connected to the oxidant gas discharge path308. A muffler 310 is disposed at the downstream end portion of theoxidant gas discharge path 308.

The third pressure regulation valve 39 is disposed in the bypass path306. The third pressure regulation valve 39 is an electromagnetic valveor an electric valve. The third pressure regulation valve 39 regulatesthe flow rate of the oxidant gas which flows through the bypass path 306with the opening degree of the third pressure regulation valve 39regulated by the control device 60. The bypass path 306 is a pipe thatconnects between the oxidant gas supply path 302 and the oxidant gasdischarge path 308 not by way of the fuel cell stack 116.

The fuel gas supply/discharge system 50 includes a fuel gas supplysystem 50A, a fuel gas circulation system 50B, and a fuel gas dischargesystem 50C.

The fuel gas supply system 50A supplies the fuel gas to the anodes ofthe fuel cell stack 116. The fuel gas supply system 50A includes a fuelgas tank 51, a fuel gas supply path 501, an open/close valve 52, aregulator 53, an injector 54, and a pressure sensor 59. The fuel gastank 51 stores a hydrogen gas at a high pressure, for example. The fuelgas supply path 501 is a pipe connected to the fuel gas tank 51 and thefuel cell stack 116 to allow the fuel gas, which is directed from thefuel gas tank 51 toward the fuel cell stack 116, to flow therethrough.The open/close valve 52 allows the fuel gas in the fuel gas tank 51 toflow downstream when the valve is open. The regulator 53 regulates thepressure of the fuel gas upstream of the injector 54 under control bythe control device 60. The injector 54 is disposed in the fuel gassupply path 501 upstream of a location where a fuel gas circulation path502 to be discussed later is merged at the fuel gas supply path 501. Theinjector 54 is an open/close valve driven electromagnetically inaccordance with a drive period and a valve opening time set by a controlsection 62, and regulates the amount of the fuel gas to be supplied tothe fuel cell stack 116. The pressure sensor 59 measures the internalpressure (supply pressure of the fuel gas) of the fuel gas supply path501 downstream of the injector 54. The measurement result is transmittedto the control device 60.

The fuel gas circulation system 50B circulates the fuel gas dischargedfrom the fuel cell stack 116 (also referred to as “fuel off gas”) to thefuel gas supply path 501 again. The fuel gas circulation system 50B hasa fuel gas circulation path 502, a gas-liquid separator 57, acirculation pump 55, and a motor 56. The fuel gas circulation path 502is a pipe connected to the fuel cell stack 116 and the fuel gas supplypath 501 to allow the fuel off gas, which is directed toward the fuelgas supply path 501, to flow therethrough. The gas-liquid separator 57is provided in the fuel gas circulation path 502, and separates liquidwater from an anode off gas in which liquid water is mixed. Thecirculation pump 55 drives the motor 56 to circulate the anode off gasin the fuel gas circulation path 502 toward the fuel gas supply path501.

The fuel gas discharge system 50C discharges the anode off gas andliquid water generated through power generation by the fuel cell stack116 to the outside. The fuel gas discharge system 50C has an airdischarge/water discharge path 504 and an air discharge/water dischargevalve 58. The air discharge/water discharge path 504 is a pipe thatcommunicates between a discharge port of the gas-liquid separator 57,which discharges liquid water, and the outside.

The air discharge/water discharge valve 58 is disposed in the airdischarge/water discharge path 504, and opens and closes the airdischarge/water discharge path 504. Examples of the air discharge/waterdischarge valve 58 include a diaphragm valve. During normal operation ofthe fuel cell system 10, the control device 60 instructs the airdischarge/water discharge valve 58 to open at a timing determined inadvance.

The refrigerant circulation system 70 includes a refrigerant circulationpath 79, a refrigerant circulation pump 74, a motor 75, a radiator 72, aradiator fan 71, and a stack temperature sensor 73.

The refrigerant circulation path 79 has a refrigerant supply path 79Aand a refrigerant discharge path 79B. The refrigerant supply path 79A isa pipe that supplies the refrigerant to the fuel cell stack 116. Therefrigerant discharge path 79B is a pipe that discharges the refrigerantfrom the fuel cell stack 116. The refrigerant circulation pump 74 isdriven by the motor 75 to feed the refrigerant in the refrigerant supplypath 79A to the fuel cell stack 116. The radiator fan 71 feeds air tothe radiator 72 to radiate heat and cool the refrigerant which flowsinside the radiator 72. The stack temperature sensor 73 measures atemperature related to the fuel cell system 10. Specifically, the stacktemperature sensor 73 measures the temperature of the refrigerant in therefrigerant discharge path 79B. The result of measuring the temperatureof the refrigerant is transmitted to the control device 60. The controldevice 60 controls operation of the fuel cell system 10 using thetemperature measured by the stack temperature sensor 73 as thetemperature of the fuel cell stack 116. The refrigerant circulationsystem 70 may include a heater that heats the refrigerant.Alternatively, in place of the outside air temperature sensor 38, thestack temperature sensor 73 may be used as the temperature sensordescribed in the SUMMARY.

FIG. 3 is a conceptual diagram illustrating the electrical configurationof the fuel cell system 10. The fuel cell system 10 includes a fuel-celldirect-current/direct-current converter (FDC) 95, adirect-current/alternating-current (DC/AC) inverter 98, a voltage sensor91, and a current sensor 92.

The voltage sensor 91 is used to measure the voltage of the fuel cellstack 116. The voltage sensor 91 is connected to each of all the fuelcells 11 of the fuel cell stack 116, and measures the voltage of each ofall the fuel cells 11. The voltage sensor 91 transmits the measurementresult to the control device 60. The total voltage of the fuel cellstack 116 is obtained by summing the voltages of all the fuel cells 11measured by the voltage sensor 91. The fuel cell system 10 may have avoltage sensor that measures voltages of both ends of the fuel cellstack 116, in place of the voltage sensor 91. In this case, the measuredvoltage values of both ends are used as the total voltage of the fuelcell stack 116. The current sensor 92 measures the value of a currentoutput from the fuel cell stack 116, and transmits the measurementresult to the control device 60.

The FDC 95 is a circuit configured as a direct-current/direct-current(DC/DC) converter. The FDC 95 controls the output voltage of the fuelcell stack 116 based on a voltage command value transmitted from thecontrol device 60. The FDC 95 also controls the output current of thefuel cell stack 116 based on a current command value transmitted fromthe control device 60. The current command value is a value as a targetvalue for the output current of the fuel cell stack 116, and is set bythe control device 60. The control device 60 generates the currentcommand value by calculating a required current value based on requiredgenerated power of the fuel cell stack 116, for example.

The DC/AC inverter 98 is connected to the fuel cell stack 116 and a load255 such as a drive motor. The DC/AC inverter 98 converts DC poweroutput from the fuel cell stack 116 into AC power to be supplied to theload 255.

The fuel cell system 10 further includes a secondary battery 96 and abattery direct-current/direct-current converter (BDC) 97. The secondarybattery 96 is constituted of a lithium-ion battery, for example, andfunctions as an auxiliary power source. In addition, the secondarybattery 96 supplies power to the load 255, and is charged with powergenerated or regenerated by the fuel cell stack 116. That is, thesecondary battery 96 is used to be charged with and discharge powergenerated by the fuel cell stack 116.

The BDC 97 is a circuit constituted as a DC/DC converter together withthe FDC 95, and controls charge and discharge of the secondary battery96 in accordance with an instruction from the control device 60. The BDC97 measures the state of charge (SOC: remaining capacity) of thesecondary battery 96, and transmits the measurement result to thecontrol device 60.

FIG. 4 is an internal block diagram of the control device 60. Thecontrol device 60 includes a storage section 68 constituted from arandom access memory (RAM) or a read only memory (ROM), and the controlsection 62. The control section 62 controls operation of the fuel cellsystem 10 based on a measured voltage value Vt measured by the voltagesensor 91, for example.

The storage section 68 stores various programs to be executed by thecontrol section 62. The control section 62 executes the various programsin the storage section 68 to function as an operation control section 64and a voltage condition determination section 66. The operation controlsection 64 controls operation of the fuel cell system 10 in accordancewith the measured voltage value Vt etc. The voltage conditiondetermination section 66 functions when a start switch of the fuel cellsystem 10 is turned on to start the fuel cell system 10 and warm-upoperation, in which the temperature of the fuel cell stack 116 israpidly raised through low-efficiency operation, is executed. Thewarm-up operation is executed when the measured value of the outside airtemperature sensor 38 indicates a temperature below the freezing point,for example. The term “warm-up operation” refers to an operation statein which the temperature of the fuel cell stack 116 is raised using heatgenerated by the fuel cell stack 116 such that the temperature of thefuel cell stack 116 reaches a target temperature (e.g. 65° C.)determined in advance as a steady state. In the warm-up operation, thestoichiometric ratio of the oxidant gas to be supplied to the fuel cellstack 116 is set to be less than the stoichiometric ratio in the steadystate, and the power generation loss of the fuel cell stack 116 isincreased by increasing the oxygen concentration overvoltage. The“stoichiometric ratio of the oxidant gas” means the ratio of the amountof actually supplied oxygen to the minimum amount of oxygen required togenerate required generated power. In the present embodiment, thestoichiometric ratio of the oxidant gas during the warm-up operation isabout 1.0. The voltage condition determination section 66 determineswhether a voltage condition determined in advance for executing thewarm-up operation by starting current sweep, which corresponds to takingout a current from the fuel cell stack 116, is met. This will bediscussed in detail later.

FIG. 5 indicates the temperature properties of the secondary battery 96.Power that can be charged to and discharged from secondary batteriessuch as lithium-ion batteries is drastically limited when thetemperature is below the freezing point, in particular −20° C. (Celsius)or lower. Consequently, when power generated by the fuel cell stack 116exceeds or falls short of required generated power, it may be difficultto charge the secondary battery 96 with the excessive power or dischargepower for supplementing the shortage from the secondary battery 96.Hence, when the measured value of the outside air temperature sensor 38indicates a temperature below the freezing point, in particular −20° C.or lower, the fuel cell system 10 is preferably controlled such that thepower generated by the fuel cell stack 116 does not significantlydeviate from the required generated power.

FIG. 6 is a flowchart illustrating a start process for the fuel cellsystem 10. FIG. 7 is a flowchart illustrating an operation pointtransition process. FIG. 8 is a first chart illustrating therelationship between the voltage (total voltage) and the current of thefuel cell stack 116 from the start of the start process until a targetoperation point Pg is reached. FIG. 9 is a second chart illustrating therelationship between the voltage (total voltage) and the current of thefuel cell stack 116 from the start of the start process until the targetoperation point Pg is reached. The dashed curve indicated in FIG. 8 isan equal power line PL that connects operation points that indicate thesame generated power as certain required generated power (e.g. requiredgenerated power at the target operation point Pg) of the fuel cell stack116. The start process indicated in FIG. 6 is triggered when the startswitch of the fuel cell system 10 is turned on.

As illustrated in FIG. 6, the control section 62 determines whetherthere is a warm-up request (step S10). In the present embodiment, thecontrol section 62 determines that there is a warm-up request when themeasured value of the outside air temperature sensor 38 indicates atemperature determined in advance or lower. The temperature determinedin advance may be the freezing point, or may be a temperature that islower than the freezing point, for example. When it is determined thatthere is no warm-up request (step S10: No), the control section 62 endsthe start process. After the start process is ended, the control section62 executes a normal power generation process in which the fuel cellstack 116 is caused to generate power in accordance with a request fromthe load 255, for example.

When it is determined that there is a warm-up request (step S10: Yes),on the other hand, the operation control section 64 starts to supply theoxidant gas, containing oxygen, to the cathode of each of the fuel cells11 by controlling the oxidant gas supply/discharge system 30, includingthe oxidant gas supply system 30A, before executing warm-up operation bystarting current sweep (step S20). Consequently, the voltage of the fuelcell stack 116 is raised until a voltage condition determined in advanceis met. In addition, in step S20, the control section 62 starts tosupply the fuel gas at a flow rate determined in advance to the anode ofeach of the fuel cells 11 by controlling the fuel gas supply/dischargesystem 50. In addition, in step S20, the control section 62 starts tocirculate the refrigerant by controlling operation of the refrigerantcirculation system 70.

The voltage condition determined in advance is set to a condition underwhich recombination of hydrogen at the cathode can be suppressed withhydrogen ions conducted from the anode to the cathode of each of thefuel cells 11 combined with oxygen existing at the cathode when warm-upoperation is executed. That is, the voltage condition determined inadvance is set to a voltage condition under which it can be determinedthat there exists oxygen enough to be combined with hydrogen ionsconducted to the cathode. In the present embodiment, the voltagecondition determined in advance is prescribed in accordance with thetotal voltage value of the fuel cell stack 116, and is a condition thatthe measured voltage value (total measured voltage value) Vt whichrepresents the total voltage value of the voltage sensor 91 has exceededa reference voltage value Vs determined in advance. The referencevoltage value Vs is Vc×Ln, for example. Vc is the cell reference voltagevalue of each fuel cell 11. Ln is the number of the stacked fuel cells11. Vc is set to a value that can be determined that sufficient oxygenhas been supplied to the cathodes of the fuel cells 11, for example,0.88 V or more. The upper limit of Vc is a value with which degradationof catalyst layers of the fuel cells 11 can be suppressed. In thepresent embodiment, Vc is set to 0.88 V.

Subsequent to step S20, the voltage condition determination section 66determines whether the measured voltage value Vt of the voltage sensor91 has exceeded the reference voltage value Vs (step S30). When themeasured voltage value Vt is equal to or less than the reference voltagevalue Vs (step S30: No), the operation control section 64 continues theprocess in step S20 without interrupting the process. When the measuredvoltage value Vt has become more than the reference voltage value Vs(step S30: Yes), on the other hand, the operation control section 64permits current sweep from the fuel cell stack 116 (step S40), andstarts the operation point transition process (step S50). That is,current sweep in the operation point transition process in step S50 isstarted when current sweep is permitted.

The operation point transition process is a process that is a part ofthe warm-up operation. As indicated by the direction of the arrows inFIG. 8, the operation point transition process is a process executedduring a period (transition period) from the start of current sweepuntil the operation point of the fuel cell stack 116 reaches the targetoperation point Pg which is determined by a target voltage value Vg anda target current value Ig for the fuel cell stack 116. The controlsection 62 sets the voltage command value and the current command valuesuch that an operation point, which is determined by the voltage valueand the current value of the fuel cell stack 116, is on the equal powerline PL, which indicates the same generated power as the requiredgenerated power of the fuel cell stack 116, when the operation point iscaused to transition in at least a part of the transition period. In thepresent embodiment, the voltage command value and the current commandvalue are set such that the operation point is on the equal power linePL when the operation point is caused to transition after the measuredvoltage value Vt has become equal to or less than a switching voltagevalue Vsw in the transition period. In another embodiment, the voltagecommand value and the current command value may be set such that theoperation point is on the equal power line PL when the operation pointis caused to transition in the entire transition period. After theoperation point transition process, warm-up operation is executed untila target temperature determined in advance is reached at the targetoperation point Pg.

Before describing the details of the operation point transition processin step S50, the content of the processes up to the permission ofcurrent sweep in step S40 will be described with reference to FIG. 9. Attime t0, it is determined that there is a warm-up request, and supply ofthe oxidant gas to the cathode of each of the fuel cells 11 is started.When the oxidant gas is supplied to the cathode, the total voltage ofthe fuel cell stack 116 is raised. In the present embodiment, the totalvoltage of the fuel cell stack 116 becomes more than the referencevoltage value Vs at time t1. Consequently, the operation pointtransition process is executed at time t1. As indicated in FIG. 8, theoperation point transition process is a process executed from the startof current sweep until the target operation point Pg is reached. Inwarm-up operation control including the operation point transitionprocess, the rotational speed of the compressor 33 (FIG. 2) ispreferably kept constant after a target rotational speed determined inadvance is reached, in order to suppress significant fluctuations inrequired generated power of the fuel cell stack 116. Hence, in thewarm-up operation control, the opening degree of the second pressureregulation valve 37 or the opening degree of the third pressureregulation valve 39 is adjusted to vary the flow rate of the oxidant gasto be supplied to the cathode after the compressor 33 reaches the targetrotational speed. In the warm-up operation control according to thepresent embodiment, the first pressure regulation valve 36 is kept fullyopen.

As illustrated in FIG. 7, the operation control section 64 executesactual voltage control in a pre-switch period of the transition period(step S52). The pre-switch period is a period until the measured voltagevalue Vt reaches the switching voltage value Vsw. In the actual voltagecontrol, the operation control section 64 sets the current command valuebased on the required generated power of the fuel cell stack 116 and themeasured voltage value Vt of the voltage sensor 91 which is the actualvoltage of the fuel cell stack 116. Specifically, the operation controlsection 64 calculates the current command value by dividing the requiredgenerated power by the measured voltage value Vt, and sets the currentcommand value. In the actual voltage control, the operation controlsection 64 performs current sweep by controlling the FDC 95 such thatthe calculated current command value is achieved.

After the actual voltage control is started in step S52, the voltagecondition determination section 66 determines whether the measuredvoltage value Vt has become equal to or less than the switching voltagevalue Vsw determined in advance (step S54). Step S52 is executed untilthe measured voltage value Vt becomes equal to or less than theswitching voltage value Vsw. When the measured voltage value Vt hasbecome equal to or less than the switching voltage value Vsw, theoperation control section 64 executes one of normal current control andstand-by control (step S56). That is, one of the normal current controland the stand-by control is executed in a post-switch period, of thetransition period, from the time when the measured voltage value Vt hasbecome equal to or less than the switching voltage value Vsw until thetime when the measured voltage value Vt reaches the target operationpoint Pg.

The stand-by control is executed with the normal current controlsuspended when a certain condition is met in the post-switch period. Theswitching voltage value Vsw is set to a value obtained by adding anaddition voltage value Vad determined in advance to the target voltagevalue Vg. The addition voltage value Vad is preferably set to a valuethat does not fall below the target voltage value Vg even when excessivecurrent sweep is caused. In the present embodiment, the addition voltagevalue Vad is set to 66 V.

In the normal current control, the control section 62 raises the currentcommand value to the target current value Ig at a proportion determinedin advance. The control section 62 suspends the normal current controland executes the stand-by control when the measured voltage value Vtreaches a control start voltage value which is smaller than the voltagecommand value. In the stand-by control, the control section 62 keeps thecurrent command value constant by holding the current command value atthe time when the measured voltage value Vt reaches the control startvoltage value. Consequently, the control section 62 ends the stand-bycontrol by permitting a change in the current command value when themeasured voltage value Vt reaches a permission voltage value, which isequal to or more than the voltage command value, by raising the voltageof the fuel cell stack 116. The control start voltage value may be setsuch that the stand-by control is executed immediately after themeasured voltage value Vt falls below the voltage command value, or maybe set to be smaller than the voltage command value by a value (e.g. 5V) determined in advance in consideration of the precision of themeasured voltage value Vt. In addition, the permission voltage value maybe the same value as the voltage command value, or may be a value thatis larger than the voltage command value by a certain value (e.g. 5 V)in consideration of the precision of the measured voltage value Vt. Inthe stand-by control, the control section 62 may increase the flow rateof the oxidant gas to be supplied to the fuel cell stack 116 byadjusting the opening degree of the second pressure regulation valve 37or the third pressure regulation valve 39 illustrated in FIG. 2.Consequently, the voltage of the fuel cell stack 116 can be raised moreefficiently. The control section 62 can resume the normal currentcontrol by permitting a change in the current command value duringexecution of the stand-by control.

It is assumed that the measured voltage value Vt reaches the controlstart voltage value Vcs, which is smaller than the target voltage valueVg as the voltage command value, at time t3 as indicated in FIG. 9. Attime t3, a measured current value It of the current sensor 92 (FIG. 3)has not reached the target current value Ig. In this case, the measuredvoltage value Vt reaches the control start voltage value Vcs at time t3,and thus the control section 62 suspends the normal current control andexecutes the stand-by control. That is, the control section 62 maintainsthe current command value at a constant value Ia by holding the currentcommand value at time t3.

At time t4, the measured voltage value Vt reaches the permission voltagevalue Vp which is equal to or more than the target voltage value Vg asthe voltage command value, and therefore the control section 62 ends thestand-by control and resumes the normal current control. Consequently,the current command value is raised again toward the target operationpoint Pg at the proportion determined in advance through the normalcurrent control. The stand-by control is also executed in a similarmanner in a period from time t5 to time t6 and a period from time t7 totime t8.

As illustrated in FIG. 7, the control section 62 determines whether theoperation point (measured current value It and measured voltage valueVt) of the fuel cell stack 116 has reached the target operation point Pg(step S58). The control section 62 executes one of the normal currentcontrol and the stand-by control until the operation point reaches thetarget operation point Pg. When the operation point reaches the targetoperation point Pg, on the other hand, the control section 62 ends theoperation point transition process. In the example illustrated in FIG.9, the operation point reaches the target operation point Pg at time t9.After the operation point transition process is ended, the controlsection 62 executes warm-up operation at the target operation point Pguntil the fuel cell stack 116 reaches the target temperature. Thecontrol section 62 determines whether a measured value of the stacktemperature sensor 73 (FIG. 2) as the temperature of the fuel cell stack116 has reached the target temperature.

With the first embodiment described above, warm-up operation can beperformed after sufficient oxygen is provided to the cathodes of thefuel cell stack 116, by performing current sweep after the measuredvoltage value Vt meets a voltage condition determined in advance.Consequently, it is possible to reduce the possibility that pumpinghydrogen is generated because of the lack of oxygen at the cathodeduring warm-up operation. By reducing the possibility that pumpinghydrogen is generated, it is possible to suppress release of hydrogen tothe outside via the oxidant gas discharge path 308. Additionally, thecontrol section 62 sets the voltage command value and the currentcommand value such that the operation point is on the equal power linePL when the operation point is caused to transition in the transitionperiod, and thus deviation of the actual power generated by the fuelcell stack 116 from the required generated power can be suppressed.Consequently, the charge/discharge amount of the secondary battery canbe controlled within a certain range.

With the first embodiment described above, in addition, the actualvoltage control is executed in the pre-switch period, and thus anincrease in the difference between the required generated power and theactual power generated by the fuel cell stack 116 can be suppressedwhile suppressing generation of pumping hydrogen. Consequently, thecharge/discharge amount of the secondary battery 96 can be furthercontrolled within a certain range.

With the first embodiment described above, in addition, the normalcurrent control is executed in the post-switch period, and thusexecution of excessive current sweep can be suppressed. With thisembodiment, additionally, the shortage of oxygen at the cathode can besuppressed by executing the stand-by control when the normal currentcontrol is executed and the measured voltage value Vt reaches thecontrol start voltage value. Thus, generation of pumping hydrogen can besuppressed.

B. Second Embodiment

FIG. 10 is a flowchart illustrating a start process for a fuel cellsystem 10 according to a second embodiment. The difference from thestart process (FIG. 6) according to the first embodiment described aboveis step S30 a. The other steps are the same between the first embodimentand the second embodiment, and therefore are given the same referencesigns to omit description. In the second embodiment, the voltagecondition which is determined in advance and under which current sweepis permitted is a condition that the measured voltage value of the endportion-side fuel cells 11 e has become more than an end portion-sidereference voltage value determined in advance.

Subsequent to step S20, the voltage condition determination section 66determines whether the measured voltage value of the end portion-sidefuel cells 11 e which is measured by the voltage sensor 91 has exceededan end portion-side reference voltage value Vce determined in advance(step S30 a). The end portion-side reference voltage value Vce is set toa value that can be determined that sufficient oxygen has been suppliedto the cathode of the end portion-side fuel cells 11 e, for example, 0.8V. When the determination in step S30 a is made based on the respectivemeasured voltage values of the plurality of end portion-side fuel cells11 e, the control section 62 determines whether the respective measuredvoltage values of the predetermined number of the end portion-side fuelcells 11 e have become more than the end portion-side reference voltagevalue Vce, for example.

With the second embodiment described above, the same effects can beachieved for having the same configuration as that according to thefirst embodiment described above. For example, warm-up operation can beperformed after sufficient oxygen is provided to the cathodes of thefuel cell stack 116, by performing current sweep after the measuredvoltage value of the end portion-side fuel cells 11 e meets a voltagecondition determined in advance. Consequently, it is possible to reducethe possibility that pumping hydrogen is generated because of the lackof oxygen at the cathode during warm-up operation. By reducing thepossibility that pumping hydrogen is generated, it is possible tosuppress release of hydrogen to the outside via the oxidant gasdischarge path 308. In addition, generation of pumping hydrogen can befurther suppressed by determining whether the voltage value of the endportion-side fuel cells 11 e which are positioned on the side of thesecond end portion meets a voltage condition determined in advance, evenif the fuel cell stack 116 is so long in the stacking direction SD thatit takes a considerable time for the oxidant gas to reach the side ofthe second end portion. That is, generation of pumping hydrogen can befurther suppressed by determining whether the measured voltage value ofthe end portion-side fuel cells 11 e meets a voltage conditiondetermined in advance, the voltage of the end portion-side fuel cells 11e being raised slowly, even when the voltage rise due to supply of theoxidant gas is slower on the side of the second end portion of the fuelcell stack 116 than on the side of the first end portion of the fuelcell stack 116 since the oxidant gas reaches the side of the second endportion later than the side of the first end portion.

C. Other Embodiments C-1. First Other Embodiment

In the first embodiment described above, the fuel gas, the oxidant gas,and the refrigerant are supplied to and discharged from only the side ofthe first end portion of the fuel cell stack 116 of the fuel cell system10. However, the present disclosure is not limited thereto. For example,the fuel gas, the oxidant gas, and the refrigerant may be supplied tothe side of the first end portion of the fuel cell stack 116 of the fuelcell system 10, and discharged from the side of the second end portion,for example.

C-2. Second Other Embodiment

The control section 62 starts current sweep when the total voltage valueof the fuel cell stack 116 meets a voltage condition in the firstembodiment described above, and starts current sweep when the voltagevalue of the end portion-side fuel cells 11 e meets a voltage conditionin the second embodiment described above. However, the presentdisclosure is not limited thereto. For example, current sweep may bestarted when the voltage value of the fuel cells 11 which are positionedon the first side of fuel cell stack 116 or the voltage value of thefuel cells 11 which are positioned at the middle meets a voltagecondition.

C-3. Third Other Embodiment

In each of the embodiments described above, the control section 62executes the actual voltage control, the normal current control, and thestand-by control in the transition period. However, the presentdisclosure is not limited thereto. For example, the control section 62may not execute the stand-by control, or may execute only one of theactual voltage control and the normal current control, in the transitionperiod. In addition, control for temporarily reducing the currentcommand value may be performed in the transition period, for example.

C-4. Fourth Other Embodiment

In each of the embodiments described above, in step S10 indicated inFIG. 6, the control section 62 determines that there is a warm-uprequest when the measured value of the outside air temperature sensor 38indicates a temperature determined in advance or lower. However, thepresent disclosure is not limited thereto. For example, the controlsection 62 may determine that there is a warm-up request when themeasured value of the stack temperature sensor 73 indicates atemperature determined in advance or lower.

The present disclosure is not limited to the embodiments discussedabove, and can be implemented in various configurations withoutdeparting from the scope and spirit of the present disclosure. Forexample, the technical features of the embodiments corresponding to thetechnical features in the aspects described in the SUMMARY field can bereplaced or combined, as appropriate, in order to solve some or all ofthe issues discussed above or achieve some or all of the effectsdiscussed above. In addition, the technical features can be deleted, asappropriate, unless such technical features are described as essentialherein.

What is claimed is:
 1. A fuel cell system comprising: a fuel cell stackthat has a plurality of stacked fuel cells each having an anode and acathode; a voltage sensor configured to measure a voltage of the fuelcell stack; an oxidant gas supply system configured to supply thecathode with an oxidant gas containing oxygen; a fuel gas supply systemconfigured to supply the anode with a fuel gas; a temperature sensorconfigured to measure a temperature related to the fuel cell system; asecondary battery configured to be charged with power generated by thefuel cell stack and discharge the power; and a control device configuredto control operation of the fuel cell system based on a measured voltagevalue measured by the voltage sensor, wherein: the control device isconfigured to, when the fuel cell system is started and a value measuredby the temperature sensor is equal to or less than a temperaturedetermined in advance, raise the voltage of the fuel cell stack until avoltage condition determined in advance is met, by causing the oxidantgas supply system to operate and supplying the cathode with the oxidantgas before current sweep from the fuel cell stack is started, andexecute warm-up operation in which a temperature of the fuel cell stackis raised, by starting the current sweep when the measured voltage valuemeets the voltage condition; and the control device is configured to,when executing the warm-up operation, set a voltage command value and acurrent command value such that an operation point determined by avoltage value and a current value of the fuel cell stack is on an equalpower line of the fuel cell stack, which indicates the same generatedpower as required generated power of the fuel cell stack, when theoperation point is caused to transition in at least a part of atransition period from the start of the current sweep until theoperation point reaches a target operation point determined by a targetvoltage value and a target current value during the warm-up operation.2. The fuel cell system according to claim 1, wherein the control deviceis configured to: execute normal current control in which the currentcommand value is raised to the target current value at a proportiondetermined in advance in a post-switch period, of the transition period,from a time when the measured voltage value has become equal to or lessthan a switching voltage value determined in advance until a time whenthe measured voltage value reaches the target voltage value of thetarget operation point; suspend the normal current control and executestand-by control in which the current command value is kept constantwhen the normal current control is executed and the measured voltagevalue reaches a control start voltage value which is less than thevoltage command value; and end the stand-by control and resume thenormal current control by permitting a change in the current commandvalue when the measured voltage value reaches a permission voltage valuewhich is equal to or more than the voltage command value duringexecution of the stand-by control.
 3. The fuel cell system according toclaim 1, wherein: the voltage sensor is configured to measure a totalvoltage of the fuel cell stack; and the voltage condition is a conditionthat a value of the total voltage as the measured voltage value measuredby the voltage sensor has become more than a reference voltage valuedetermined in advance.
 4. The fuel cell system according to claim 1,wherein: supply of the oxidant gas to the fuel cell stack is performedon a side of a first end portion of the fuel cell stack in a stackingdirection in which the fuel cells are stacked; the voltage sensor isconfigured to measure a voltage of an end portion-side fuel cell whichis a fuel cell among the fuel cells, the end portion-side fuel cellbeing positioned on a side of a second end portion which is oppositefrom the side of the first end portion; and the voltage condition is acondition that a voltage value of the end portion-side fuel cell as themeasured voltage value measured by the voltage sensor has become morethan an end portion-side reference voltage value determined in advance.